Asynchronous transfer mode (ATM) technology is rapidly becoming the technology of choice for broadband digital communications. The fixed length ATM cell is well suited to the transport of multi-media communications including voice, video and data. As a result, ATM technology supports a wide range of services and applications. The ability of ATM technology to adapt to existing systems and architectures results in improved performance and bandwidth utilization.
This has resulted in the integration of ATM architectures with other developing technologies in order to gain the advantages offered by ATM.
The rapid expansion of Internet connections and services in recent years has resulted in attempts to increase the physical layer transfer rate, otherwise referred to herein as the transport rate, over existing telephone connections. Although coaxial cable or optical fiber is being used for trunk connections between central offices, connections to private homes and small businesses generally rely on existing local copper loops. These, of course, have a limited bandwidth and attempts to improve utilization of this bandwidth are being investigated constantly.
One of the technologies which will improve transport rate over that obtained by presently used modems and ISDN lines is known as a digital subscriber line (DSL). xDSL technologies include symmetric digital subscriber line (SDSL), asymmetric digital subscriber line (ADSL) and very-high rate digital subscriber line (VDSL). In ADSL the asymmetric line provides a higher transport rate in the downstream (i.e. to the ADSL subscriber) direction than it does in the upstream (i.e. from the ADSL subscriber) direction. The range of downstream transport rate typically varies from as little as 32 kb/s to over 8 mb/s and the upstream rate from 32 kb/s to over 640 kb/s. This downstream rate offers considerable improvement over the 160 kb/s rates provided by existing ISDN architectures.
As the potential transport rate increases, minor variations in the local loop as a function of time become significant. The transport rate within the above-mentioned range in xDSL technology is known to vary due to physical conditions of the local loop. These physical conditions include the actual condition of the loop itself, temperature variations and/or electromagnetic interference. Therefore, conventional xDSL termination units, also commonly referred to as xDSL modems, are equipped with dynamic rate adaptation functionality whereby the modem dynamically adjusts its transmission rate according to the measured physical characteristics (i.e., usable transport rate) of the loop. The transmission rate in use by the xDSL modem is stored in an internal register which may be read by an end-system connected to the modem.
Currently, the proposed broadband ADSL service interface with an ATM network utilizes unspecified bit rate (UBR), one of the five categories of service defined in the ATM Forum's Traffic Management Specification, version 4.0. This service category is intended for non-real time applications such as those that do not require tightly constrained delay and delay variations. UBR service does not specify traffic related service guarantees. Under UBR service a peak cell rate (PCR) is negotiated between the source and destination at connection setup. The source can send data up to the negotiated PCR.
As indicated previously, the transport rate characteristics of the xDSL link forming part of the path between the two end-systems may change during the time of the connection (i.e. after the connection has been set up). If, for example, the transport rate decreases during this interval the xDSL link will not be able to carry the data at the negotiated transmission rate of the source. As a result, congestion will occur at the interface between the ATM path and the xDSL link leading to packet discard and a reduction in system performance.
Another network architecture in which a transmission link may have a variable transport rate is in a wireless configuration. In this example, the data is transmitted from the ATM network interface card to the destination across a wireless link. Such wireless links are subject to transport rate variations due to atmospheric conditions changing path lengths, etc. Again, if the transport rate characteristic is reduced after the connection has been negotiated, congestion will occur.
A third example of a transmission link having a dynamically varying transport rate is an inverse multiplexing over ATM (IMA) system. In this example, one of the multi paths through the network may experience a temporary failure which results in increase demands on the bandwidth requirements of the remaining paths. If the connections are already operating at or near their physical capacity they will be unable to satisfy the negotiated peak cell rate (PCR) resulting in congestion.
There is, therefore, a requirement to improve the management of data traffic through an ATM path that includes a link having a dynamically variable physical layer transport rate.